Rechargeable lithium batteries, which are attractive as power sources for portable electronics, use organic electrolyte, exhibit twice the discharge capacity of conventional batteries with alkaline aqueous solution electrolyte and have higher energy density than conventional batteries.
Negative active materials for rechargeable lithium batteries are made from materials which are capable of intercalating and deintercalating lithium. Examples of such materials include carbonaceous materials such as graphite, e.g. artificial graphite, natural graphite, or hard carbon. A lower discharge voltage to lithium, i.e. −0.2V from graphite, renders a higher discharge voltage of 3.6V which provides advantages in energy density, and good reversibility guarantees long cycle life.
Positive active materials for rechargeable lithium batteries are also made of material capable of intercalating lithium. Examples include oxides of lithium and transition metals such as LiCoO2, LiMn2O4, and LiNiMO2 where M is a divalent or trivalent metal. Such an oxide of lithium and a transition metal may be prepared by mixing a lithium compound such as lithium carbonate or lithium hydroxide, a cobalt compound such as lithium oxide or lithium carbonate, or a nickel compound such as NiM(OH)2 or an oxide thereof at a mixing ratio of Li/Co or Li/Ni of approximately 1:1, and sintering the resulting mixture at 600 to 1000° C. for 7 to 25 hours. However, the procedure generates unreacted lithium compounds on the surface of the resulting product, which makes a base positive active material composition, resulting in gelation problems. Furthermore, the unreacted lithium compounds react with CO2 in the ambient atmosphere to generate lithium carbonate which decomposes at high temperatures and generate gas, thereby expanding battery volume and causing a swelling phenomenon. The swelling phenomenon is partly manifested by gas generated due to decomposition of the electrolyte at high temperatures in the battery.
In particular, the swelling phenomenon is severe for nickel-based active materials. This is why a nickel-based active material are not often used despite exhibiting higher capacity and costing less than the more widely used cobalt-based active materials.
Several attempts to reduce the swelling phenomenon have been suggested. Japanese Patent Laid-Open No. Hei. 5-266889 discloses controlling the mixing ratio of Co/Li or Ni/Li to less than 1 to reduce the amount of lithium carbonate produced. However, this procedure produces an unreacted Co or Ni compound, decreasing capacity.
Japanese Patent Laid-Open No. Hei. 10-79244 discloses that a neutralized salt of an organic acid and/or an inorganic acid may be added to a paste containing active materials and a binder to prevent the paste from gelling. However, prior to the achievement of the effect, the active material absorbs CO2 or water, so this is not a satisfactory means for solving the gassing problems.
Japanese Patent Laid-Open No. 2003-123755 discloses that a lithium-containing oxide compound having neutral lithium salt on its surface, is prepared by a method including: washing, filtering and drying the powder of a basic lithium-containing compound oxide with an acidic aqueous solution; spraying the acidic aqueous solution onto the powder of the lithium-containing compound oxide and drying the same; and spraying an acidic gas to the powder of the lithium-containing compound oxide. However, the process partially coats the surface of the lithium-containing compound oxide active material and reduces the amount of lithium that can participate in the charge and discharge, thereby deteriorating the electrochemical characteristics, such as high rate characteristics.